This application claims the priority of German Patent Application No. 197 47 671.6, filed Oct. 29, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method for operating a multicylinder internal combustion engine, such as a motor vehicle engine with at least one adsorber catalytic converter in the exhaust line of the internal combustion engine. The adsorber catalytic converter is operated with periodically alternating adsorption and desorption steps. In particular, the invention relates to methods in which (1) the exhaust gas leaving the adsorber catalytic converter in desorption operation is recycled to the intake side of the internal combustion engine and/or (2) the exhaust is passed through an oxidation catalytic converter connected upstream before it enters the adsorber catalytic converter.
German Patent document DE 195 17 168 A1 discloses an NO.sub.x adsorber catalytic converter that operates alternately in adsorption and desorption operation. In desorption operation of the NO.sub.x adsorber catalytic converter, incomplete combustion is produced in the internal combustion engine. For this purpose, exhaust recycling to the intake manifold of the internal combustion engine is provided, thereby reducing the combustion temperature in the engine and/or the likelihood of increasing misfirings in the engine. By such exhaust recycling (or possibly through other measures, particularly retarding the ignition point, thus making the air/fuel mixture in the engine leaner, or intermittently inhibiting ignition in all or individual cylinders), the combustion in an internal combustion engine operating with spark ignition is made worse, so that incompletely burned fuel components enter the exhaust from the internal combustion engine and make possible sufficient desorption and reduction of the stored nitrogen oxides in the NO.sub.x adsorber catalytic converter during desorption operation.
German Patent document DE 195 22 165 A1 discloses a method for operating an internal combustion engine with a NO.sub.x adsorber catalytic converter in which temporary heating of the NO.sub.x adsorber catalytic converter above normal operating temperature is provided to remove impurities. For this purpose, it is proposed that the exhaust temperature be raised by measures inside the engine. In particular, fuel is afterburned in the exhaust line as both unburned fuel and excess air are fed into the exhaust from the internal combustion engine. This is achieved by incomplete combustion in one set of the engine cylinders and by combustion with excess air in another set of the engine cylinders. Moreover, retarding the ignition point and simultaneously increasing the quantity of intake air are proposed, which also leads to an increase in the exhaust temperature due to afterburning in the exhaust line. Richer operation of the engine with simultaneous addition of secondary air to the NO.sub.x adsorber catalytic converter is also proposed, with the same effect.
German Patent document DE 43 19 294 C1 discloses a method for reducing nitrogen oxides in the exhaust of an engine in which a NO.sub.x adsorber catalytic converter is operated with periodically alternating adsorption and desorption operation and the exhaust leaving the adsorber catalytic converter operating in desorption operation is recycled to the intake air of the engine. In particular, two catalytic converters in parallel can be used, operated alternately in adsorption and in desorption operation. The nitrogen oxides contained in the recycled exhaust are reacted in the engine, which can therefore be operated continuously with complete combustion.
The object of the present invention is to create a method for operating a multicylinder internal combustion engine with at least one adsorber catalytic converter, which ensures improved exhaust scrubbing by the simplest possible means with minimal impairment of engine operation.
In an embodiment of the present invention, exhaust recycling takes place in only one set of the cylinders of the internal combustion engine, which is operated during each catalytic converter desorption phase in a state of incomplete combustion. In this set of cylinders, the atmosphere is favorable for reacting desorbed nitrogen oxides. At the same time, exhaust is produced, which in turn favors desorption of the nitrogen oxides and possibly other adsorbed substances in the adsorber catalytic converter. Such thermodynamically unfavorable conditions remain limited to this set of cylinders, while the remaining cylinders can be operated with complete combustion so that the engine as a whole can achieve relatively high efficiency.
In another embodiment of the present invention, an oxidation catalytic converter connected upstream of the adsorber catalytic converter is used to process the exhaust fed to it. With the aid of rich afterburning in the oxidation catalytic converter, incompletely oxidized exhaust components are generated and favorable conditions are created for desorption in the adsorber catalytic converter. The oxidation catalytic converter is operated for short regeneration phases at an elevated temperature, where soot, particles, or hydrocarbons that formerly were precipitated onto the oxidation catalytic converter are removed. The oxidation catalyst can be regenerated either during the adsorption or the desorption operation of the adsorber catalytic converter connected downstream depending on which substances are released when the oxidation catalyst is regenerated.
In another embodiment of the present invention, the desorption process is favored in the adsorber catalytic converter by supplying exhaust low in nitrogen oxides, which comes from the set of the cylinders of the internal combustion engine involved in exhaust recycling, which is specifically adaptable to this process.
In another embodiment of the present invention, the exhaust from the cylinders to which recycled exhaust has not been fed (and possibly residual exhaust from the remaining cylinders, which are operated in a state of incomplete combustion) is fed to the adsorber catalytic converter that is adsorbing at the time, so that when it is released to the environment it is as clean as possible.
In another embodiment of the present invention, an exhaust temperature higher than that in normal operation is achieved by measures internal to the engine. The internal engine measures however usually have to be effected in only some of the cylinders. These engine control measures can be applied specifically for a short period of time, when limitation to one set of cylinders leads to less impairment of performance and engine efficiency. Moreover, the increase in exhaust temperature can be precisely adjusted and controlled.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.